PCBs are organic compounds composed of two benzene rings that are joined and containing at least 1 to 10 chlorine atoms. There can be 209 theoretically different combination of PCBs, of which 130 are found commercially.
PCBs are a group of chlorinated polyaromatic compounds including chlorinated polycyclic aromatic hydrocarbons that became widely used in numerous commercial applications starting in the early 1930s because of their unique thermal stability and non-reactive nature. It has recently been discovered that PCBs are also highly toxic. Because of this toxicity, the manufacture and use of PCBs has been greatly curtailed and their use has generally been limited to closed systems. However, because of their thermal and chemical stability and non-reactive nature, PCBs exist in significant amounts in the environment and have found their way into all levels of the food chain. There is therefore a need for an efficient and economic method for removing PCBs and other polyaromatic compounds from the environment.
Some basic PCB properties are: 1) it is odorless, tasteless, clear to pale yellow viscous liquid but when the chlorination is higher, the viscosity increases and the color becomes deeper yellow; 2) in water, it has a very low solubility, which increases with organic solvents and fats. The flash point temperature is between 170 to 380° C.; 3) it is resistant to oxidation, reduction, addition, elimination and electronic substitution reactions; 4) as the percentage of chlorine increases, the melting point and lipophilicity, which is the ability to dissolve in fats, also increases but at the same time its water solubility rate decreases; 5) PCBs can very easily penetrate skin, PVC and latex but is impermeable to butyl rubber, nitrile rubber, neoprene, and polyvinyl acetate; 6) it is very stable and does not decompose very easily and; 7) chemical, thermal and biochemical reactions are difficult because it generates very toxic products such as dibenzoidoxin and dibenzofurans with oxidation.
PCBs have been used in several applications such as: Coolants in insulation fluids for transformers, in fluorescent and electrical transformers, paints, cements, pesticides, protective covers in electrical wiring, caulking sealants, adhesives, waterproofing compounds, flame retardant products, several heavy industrial oils, electric fluids in transformers, capacitors, de-dusting agents, cutting oils, heat transfer fluids, hydraulic lubricants, asphalt roofing materials, carbonless copy paper, surgical implants, compressor oil, dielectric fluid, dyes, electromagnets, grout, inks, mixed with asbestos, natural gas pipelines, pesticides, plasticizers, rubberizes, space heaters, submersible well pumps, tar paper and wax extenders. PCBs have also been known to be by products of cigarette smoke.
PCBs can stay in the body for long periods after being absorbed by the fat cells, and also can be transmitted to the newborns through breast feeding. The ill effects of PCBs have been observed in liver damage, dental decay, rashes, irregular menstrual cycles, still-born fetuses, unusual skin sores and cancer.
Exposure to PCBs can also cause cancer, heart disease, reproductive problems, reduced sperm count, birth defects, immune suppression and endocrine disruption. For pregnant women, there is a special risk of defects in unborn and newborn child. These defects can include but are not limited to, lower birth rates, smaller head circumference, premature birth, depressed responsiveness, impaired visual recognition, poor short term memory, weight gain deficits, reduced IQ and/or difficulty paying attention.
Reports on the occurrence of PCBs in fish, mussels, seals, sea birds and birds of prey first appeared in 1966. In 1967, PCBs were detected in human adipose tissue, albeit in low concentrations. In 1968, PCBs from a leaking cooling system contaminated a rice oil tank at a food factory in Japan. As a result of the consumption of the contaminated rice oil, 1,000 people fell ill with a disease subsequently known worldwide as Yusho Disease.
Some of the methods by which PCBs can be decomposed include: 1) Incineration at a temperature of 1200° C. with excess oxygen and fuel; 2) Ultrasound where water undergoes thermolysis oxidation of the PCBs reducing it to Carbon Dioxide, Carbon Monoxide and hydrocarbons (CH+) and releasing the Chlorine (Cl−); 3) Irradiation wherein the PCBs are mixed with mineral oil or isopropyl alcohol with potassium hydroxide and then bombarded with gamma rays, releasing the Chlorine from the biphenyl; 4) Pyrolysis which uses a plasma arc process to achieve the 1200° C. but with an inert environment; 5) Use of microbes to attack the Carbon element in the PCBs; 6) Use of Enzymes as a catalyst to speed up the decomposition of the PCBs and 7) Chemical substitution to replace the Chlorine with polyethylene glycol, which must be done under nitrogen to accommodate the proper reaction which yields an insoluble aryl polyglycol that is filtered out. However, these methods are expensive and difficult to perform on a large scale.
Conventionally, PCBs were disposed of solely by incineration. However, this method is very likely to produce harmful materials (e.g., dioxins) such as by-products and, therefore, incineration disposal is not employed at present. Thus, there is a need to develop a process for decomposing PCBs that is not expensive and does not produce any harmful byproducts. In an attempt to find a process that produces no harmful materials (e.g., dioxins), there has recently been proposed a process wherein PCBs are oxidatively decomposed in supercritical water having a temperature of 374° C. or above and containing an oxidizing agent. However, since PCBs are chemically stable, it would be preferable to decompose PCBs at a temperature of 600° C. or higher.